In a hard disk drive, a magnet write transducer or head is used to write and thus store information as magnetic bits on a spinning magnetic disk. The magnetic bits are regions on the magnetic disk with a net magnetization and having north and south poles where a magnetic field exits or enters the magnetic bits. During the writing of the magnetic bits, the magnetic write head is positioned in proximity of the spinning magnetic disk. More precisely, the magnetic write head is mounted on a slider that flies over the spinning magnetic disk on an airbearing. The slider is kept over an appropriate track of the magnetic disk by a servo control system. The magnetic bits, and thus the information, is read by positioning a magnetic read transducer or head in proximity above the spinning magnetic disk and over the appropriate track by the same slider and servo control system. The magnetic field associated with the magnetic bits outside of the magnetic disk (henceforth called the external magnetic field) enters the magnetic read head and affects a magnetic sensor in the magnetic read head such that a measurable output corresponding to the magnetic bits is produced. Magnetic sensors based on the fundamental principles of magnetoresistance including anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) or spin valve and spin tunneling have been well known in the art for some time. Magnetic read heads incorporating these magnetic sensors have also been produced and widely used. For examples, see U.S. Pat. Nos. 5,159,513 and 5,206,590.
The areal density of the magnetic disk corresponds to the number of magnetic bits per unit area. There is an ongoing demand for storing more information on a given disk and thus for increasing the areal density. Magnetic scaling is a well-known approach in the art for achieving higher areal density while maintaining the signal-to-noise ratio that is ultimately necessary to obtain the measurable output from the magnetic read head corresponding to the magnetic bits. For example, according to the magnetic scaling approach the dimensions associated with magnetic recording, such as the thickness of various layers in the magnetic read head, need to be reduced as the areal density is increased. As discussed below, however, not all the consequences of the magnetic scaling approach are fully appreciated in the art.
FIG. 1 shows an illustration of a prior art magnetic read head based on a typical GMR magnetic sensor as seen from the airbearing surface. The magnetic sensor has a high coercivity ferromagnetic pinned layer 112 (such as an alloy of NiFe) with a net magnetization whose direction pointing into the page is fixed and a low coercivity ferromagnetic free layer 116 (such as an alloy of NiFe) with a net magnetization whose direction is moveable, rotating from pointing into the page to pointing out of the page in response to the external magnetic field from the magnetic disk. The direction of the magnetization in the ferromagnetic pinned layer 112 is fixed by exchange coupling with an antiferromagnetic layer 110. For a current 113 in-plane (CIP) magnetic sensor such as that shown in FIG. 1, the ferromagnetic pinned layer 112 and the ferromagnetic free layer 116 are separated by a thin film of copper 114 or other non-magnetic metal with a relatively long electron mean free path. The variation in the resistance of the GMR magnetic sensor in response to the rotation of the direction of the magnetization in the ferromagnetic free layer 116 is known in the art. It is this variation that gives rise to the measurable output from the magnetic sensor in the magnetic read head corresponding to the magnetic bits written on the magnetic disk.
An important concern in the design of the magnetic sensor in FIG. 1 is a longitudinal bias magnetic field applied to the ferromagnetic free layer 116 by the high coercivity hard magnet 118 at the two side edges of the ferromagnetic free layer 116. Longitudinal direction is the direction in the plane of the airbearing surface and parallel to the layers of the magnetic sensor, i.e., from right to left in FIG. 1, as indicated by the arrow 120. The longitudinal bias magnetic field is essential to proper operation of the magnetic sensor by ensuring that the ferromagnetic free layer 116 has a single magnetic domain. In the absence of the longitudinal bias magnetic field, the magnetic moments in the ferromagnetic free layer 116 tend to establish a magnetic multi-domain state. As is known in the art, when the ferromagnetic free layer 116 is allowed to have more than one magnetic domain it experiences Barkhausen jumps and other magnetic domain reorientation phenomena during magnetic reversal when the magnetic sensor is responding to the external magnetic field from the magnetic disk. This situation is highly undesirable since it produces noise and lowers the signal-to-noise ratio of the magnetic sensor and thus the ability to produce the measurable output corresponding to the magnetic bits.
A variety of schemes have been employed to provide the longitudinal bias magnetic field and prevent Barkhausen noise. FIG. 1 illustrates one of the more common approaches, so-called hard bias associated with the hard magnet 118 on either side of the ferromagnetic free layer 116. For more details on hard bias see U.S. Pat. No. 5,729,410.
In the course of manufacturing magnetic sensors, such as that shown in FIG. 1, it is common for the hard magnet 118 that is used to provide the hard bias to taper at the interface with the ferromagnetic free layer 116. As shown in FIG. 1, this taper produces a tip 122 of the hard magnet 118 on either side of the ferromagnetic free layer 116. Regions such as the tip 122 of the hard magnet 118 have negative consequences for the performance and the scaling of the magnetic sensor.
It is known in the art that superior materials for the hard magnet 118 should exhibit high coercivity, high remnant magnetization and the magnetic c-axis should be confined parallel to the film plane (henceforth called in-plane) as opposed to perpendicular to the film plane (henceforth called out-of-plane). These properties strongly depend on the microstructural characteristics of the hard magnet 118, which are in turn sensitive to growth conditions, film thickness and the ancillary non-magnetic layers (so-called seed layers or underlayers) onto which said hard magnet 118 is deposited. Achieving confinement of the magnetic axis in-plane is challenging and difficult in particular for very thin films in which the crystallographic growth is strongly dominated by early stages of nucleation. This situation is encountered at the tip 122 between the hard magnet 118 and the ferromagnetic free layer 116 and is also a general consequence of magnetic scaling, which dictates progressively smaller dimensions including thickness 124 of the hard magnet 118.
Alloys of CoPt and CoPtCr grown on suitable materials offer a partial solution to this hard bias challenge and are widely used as the hard magnet 118. If high temperatures are used during the deposition of the CoPtCr, grains with in-plane c-axis crystallographic orientation can be more easily obtained. Unfortunately, such high temperatures are incompatible with many of the other materials and techniques used to manufacture magnetic sensors and magnetic read heads. As a consequence, some fraction of the magnetic grains in CoPtCr films used to provide hard bias in magnetic sensors have out-of-plane c-axis crystallographic orientation.
These grains with out-of-plane c-axis crystallographic orientation degrade the magnetic sensor performance. The problem is worsened as the dimensions of the magnetic sensor are reduced per the magnetic scaling approach on account of the superparamagnetic effect which results in a loss of the magnetic order when the magnetic grain volume drops below a critical value. In addition, unlike grains in the magnetic disk, which are magnetically decoupled from one another, there is strong exchange coupling between the grains in the hard magnet 118. Furthermore, the average grain size in the hard magnet 118 is not typically scaled as the areal density is increased or, if it is decreased, the scaling ratio is larger than that dictated by the magnetic scaling approach. The combination of these effects increases the negative effect of grains in the hard magnet 118 with out-of-plane c-axis crystallographic orientation. This is especially the case in regions like the tip 122 where the number of grains is reduced as the overall magnetic sensor dimensions are scaled. Even if the average fraction of grains with out-of-plane c-axis crystallographic orientation remains fixed, the tip 122 may have a higher local fraction due to statistical fluctuations. These grains can act as nucleation sites for undesirable magnetic domains in the ferromagnetic free layer 116 with the deleterious effects described above.
One potential solution to this challenge is a seed layer, which improves the crystallographic properties of the hard magnet 118. FIG. 2 shows an illustration of seed layer 126 that helps control the crystallographic orientation of grains in the hard magnet 118. Cubic-titanium tungsten (see U.S. Pat. No. 6,278,595), a bi-layer of tantalum-oxide and Cr (see U.S. Appl. No. 2003/0058586 A1), Cr and alloys of CrMo have been used as the seed layer 126.
Recent advances, however, in the magnetic sensor in magnetic read heads have made a simple seed layer, such as seed layer 126, undesirable. In particular, the so-called ultra contiguous junction (UCJ) arrangement in the magnetic sensor. As shown in FIG. 3, in the UCJ arrangement the hard magnet 118 that provides hard bias is collinear with the ferromagnetic free layer 116 thereby avoiding magnetic instabilities in the magnetic sensor. This, in turn, requires seed layer thickness 128 be increased up to around 15–25 nm. Since the seed layer is polycrystalline, at this thickness stress and crystallographic imperfections will degrade the ability of the seed layer 126 to improve the c-axis crystallographic orientation of the grains in the hard magnet 118. This problem is illustrated in FIG. 4, which shows measured x-ray intensity as a function of diffraction angle (twice the angle of incidence as measured from the normal to the film) at grazing incidence (which is sensitive to grains with out-of-plane c-axis crystallographic orientation) for two samples that are representative of current hard magnet 118 materials used for hard bias in magnetic sensors. The first sample has CO3Pt hard magnet 118 with thickness 124 of 7.6 nm and a CrMo seed layer 126 with seed layer thickness 128 of 12.0 nm. The x-ray diffraction data 152 for the first sample is shown in FIG. 4. The second sample has 3.0 nm thick Rh cap layer on CoPtCr hard magnet 118 with thickness 124 of approximately 18.0 nm and Cr seed layer 126 with seed layer thickness 128 of 10.0 nm. The x-ray diffraction data 162 for the second sample is also shown in FIG. 4. The presence of peaks corresponding to the <11{overscore (2)}0> direction in CO3Pt 170 and CoPtCr 180 are indicative of grains with out-of-plane c-axis crystallographic orientation.
In light of this discussion, there is a need to improve the crystallographic orientation of the grains in the hard magnet 118 that provides the longitudinal bias magnetic field to the ferromagnetic free layer 116 in magnetic sensors. Furthermore, there is a need to provide this improved crystallographic orientation of the grains in the hard magnet 118 with a relatively large thickness 128 seed layer 126, such as is required in magnetic sensors with the UCJ arrangement.